Directional overcurrent relays are widely used for the protection of power distribution systems such as radial and ring sub-transmission systems and other distribution systems. These relays have a functionality enabling them to determine a fault direction. Here, a fault generally means an overcurrent, such as from a short circuit. Further, the fault direction is in most cases binary information, indicating whether the fault is a forward fault or a backward fault. In a power line connecting an upstream power source to a downstream power distribution system portion (with the normal power direction from upstream to downstream), the forward direction is downstream of the relay, and the backward, or reverse, direction is upstream of the relay.
In smart grids, decentralized or distributed units can feed power into the grid or consume power from the grid. Thus, in smart grids, the power flow direction can change with time. In this situation, “forward” and “reverse” can still be defined as above with respect to the current power flow, so that, for example, the forward direction will change if the power flow is reversed.
More generally, the fault direction is an indicator at which side of a measurement location the fault has occurred. In the above example, there are two directions, forward and backward. If the measurement location is at a node of the power network having more than two sides, there can be more than just a forward or a backward direction. For example, for a node to which one backward line portion and two forward line portions are connected, the fault direction can include the cases “forward-1”, “forward-2”, and “backward”.
The directional information provides more detailed information about the location at which a fault has occurred. This information can be used to deactivate a smaller portion of the power distribution system in the case of a fault. For example, a known ring-main feeder (e.g. for domestic supplies) has breakers at its T-junctions. If there is a fault in any of the lines of this ring-main feeder, the whole line section may be interrupted. This situation can be improved when more detailed fault directional information is obtained. For this purpose, directional overcurrent relays can be installed in the line along with breaking switches. With such a relay-switch system, a reference voltage measurement allows for the computation of the fault current and its direction. The directional information can then be used to disconnect only the appropriate section, rather than the whole line.
Known directional overcurrent relays rely on a reference voltage phasor, also known as “voltage polarization”, for estimating the direction of the fault. When a fault occurs, the fault current has a characteristic phase angle relative to the voltage phasor, where the phase angle depends on the fault direction. The fault direction is determined by comparing the current phasor (complex current value whose real part is the actual AC current) to a reference voltage phasor (industrially termed as ‘voltage polarization’) measured at a measurement location on the power line. This requires measurement of both current and voltage. This approach becomes unreliable when the fault is in close proximity to the relay, because in this case, the relay is almost grounded by the short circuit (industrially termed as ‘close-in faults’).
Further, overcurrent relays including a voltage measurement unit are expensive. Since they have to be used in large number for the above arrangement, this is a major cost factor.
In “Fault Direction Estimation in Radial Distribution System Using Phase Change in Sequence Current” of Pradhan A. K. ET AL, IEEE TRANSACTIONS ON POWER DELIVERY, IEEE SERVICE CENTER, New York, N.Y., US, vol. 22, no. 4, 1 Oct. 2007 (2007-10-01), pages 2065-2071, XP011191870, ISSN:0885-8977, DOI:10.1109/TPWRD.2007.905340, a fault direction estimation is proposed based on a phasor estimation. Time-domain current data indicative of the measured current are not obtained.
DE 19835731A1 discloses an experimental setup for fault direction estimation analyzing the phase angle between current and voltage in an electrical grid.
In “Evaluation of a New Current Directional Protection Technique Using Field Data” of Eissa M. M., IEEE TRANSACTIONS ON POWER DELIVERY, IEEE SERVICE CENTER, New York, N.Y., US, vol. 20, no. 2, 1 Apr. 2005 (2005-04-01), pages 566-572, XP011129251, ISSN: 0885-8977, D01:10.1109/WRD.2005.844356, the difference in phase relationship created by a fault is exploited by making relays which respond to phase angle difference between two quantities—such as the fault current and the prefault current.